You could be forgiven for thinking the television images in the experiment were from 2006. They were really from 1982: Israeli forces were clashing with Arab militants in Lebanon. The world was watching, charges were flying, and the air was thick with grievance, hurt and outrage.

There was only one thing on which pro-Israeli and pro-Arab audiences agreed. Both were certain that media coverage in the United States was hopelessly biased in favor of the other side.

The endlessly recursive conflict in the Middle East provides any number of instructive morals about human nature, but it also offers a psychological window into the world of partisan behavior. Israel's 1982 war in Lebanon sparked some of the earliest experiments into why people reach dramatically different conclusions about the same events.

The results say a lot about partisan behavior in general -- why Republicans and Democrats love to hate each other, for example, or why Coke and Pepsi fans clash. Sadly, the results also say a lot about the newest conflicts between Israel and its enemies in Lebanon and the Palestinian territories, and why news organizations are being besieged with angry complaints from both sides.

Partisans, it turns out, don't just arrive at different conclusions; they see entirely different worlds . In one especially telling experiment, researchers showed 144 observers six television news segments about Israel's 1982 war with Lebanon.

Pro-Israeli viewers, who watched the very same clips, spotted 16 references that painted Israel positively and 57 references that painted Israel negatively.

Both groups were certain they were right and that the other side didn't know what it was talking about.

The tendency to see bias in the news -- now the raison d'etre of much of the blogosphere -- is such a reliable indicator of partisan thinking that researchers coined a term, "hostile media effect," to describe the sincere belief among partisans that news reports are painting them in the worst possible light.

Were pro-Israeli and pro-Arab viewers who were especially knowledgeable about the conflict immune from such distortions? Amazingly, it turned out to be exactly the opposite, Stanford psychologist Lee D. Ross said. The best-informed partisans were the most likely to see bias against their side.

Ross thinks this is because partisans often feel the news lacks context. Instead of just showing a missile killing civilians, in other words, partisans on both sides want the news to explain the history of events that prompted -- and could have justified -- the missile. The more knowledgeable people are, the more context they find missing.

Even more curious, the hostile media effect seems to apply only to news sources that strive for balance. News reports from obviously biased sources usually draw fewer charges of bias. Partisans, it turns out, find it easier to countenance obvious propaganda than news accounts that explore both sides.

"If I think the world is black, and you think the world is white, and someone comes along and says it is gray, we will both think that person is biased," Ross said.

The experiment, of course, did not address whether news reports were in fact biased -- who would decide? -- or how the media ought to cover conflicts. Partisans argue that assigning equal weight to both sides is wrong when one side (theirs) is right. In any event, psychologists such as Ross are less interested in rating the news or in which side is right than in the curiosities of human perception: Why are partisans invariably blind to how news coverage might help their side?

If someone says several nice things about you and one derogatory thing, what sticks in your mind? People who are deeply invested in one side are quicker to spot and remember aspects of the news that hurt than they are to see aspects that help, said Richard Perloff, a Cleveland State University political communication researcher.

Perloff elicited the same clashing perceptions of bias from pro-Israeli and pro-Arab audiences when he showed them news clips with equal amounts of violence.

Ross and Perloff both found that what partisans worry about the most is the impact of the news on neutral observers. But the data suggest such worry is misplaced. Neutral observers are better than partisans at seeing flaws and virtues on both sides. Partisans, it turns out, are particularly susceptible to the general human belief that other people are susceptible to propaganda.

"When you are persuaded by something, you don't think it is propaganda," Ross said. "Israelis know they see the world the way they do because they are Israelis, and Arabs, too. The difference is people think in their case, their special identities are a source of enlightenment, whereas other people's source of enlightenment is a source of bias."

Next Monday: What brain-imaging research shows about partisan behavior.